Light emitting device having light extraction structure and method for manufacturing the same

ABSTRACT

A nitride-based light emitting device capable of achieving an enhancement in emission efficiency and an enhancement in reliability is disclosed. The light emitting device includes a semiconductor layer, and a light extracting layer arranged on the semiconductor layer and made of a material having a refractive index equal to or higher than a reflective index of the semiconductor layer.

This application is a Continuation of co-pending application Ser. No.11/797,727 filed on May 7, 2007, and for which priority is claimed under35 U.S.C. §120; and this application claims the benefit of Korean PatentApplication No. 10-2006-0041006, filed on May 8, 2006, Korean PatentApplication No. 10-2007-0037414, filed on Apr. 17, 2007, Korean PatentApplication No. 10-2007-0037415, filed on Apr. 17, 2007, and KoreanPatent Application No. 10-2007-0037416, filed on Apr. 17, 2007. Allthese applications are hereby incorporated by references as if fully setforth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device and a methodfor manufacturing the same, and more particularly, to a light emittingdevice capable of achieving an enhancement in emission efficiency and anenhancement in reliability and a method for manufacturing the same.

2. Discussion of the Related Art

Light emitting diodes (LEDs) are well known as a semiconductor lightemitting device which converts current to light, to emit light. Since ared LED using GaAsP compound semiconductor was made commerciallyavailable in 1962, it has been used, together with a GaP:N-based greenLED, as a light source in electronic apparatuses, for image display.

The wavelength of light emitted from such an LED depends on thesemiconductor material used to fabricate the LED. This is because thewavelength of the emitted light depends on the band-gap of thesemiconductor material representing energy difference betweenvalence-band electrons and conduction-band electrons.

Gallium nitride (GaN) compound semiconductor has been highlighted in thefield of high-power electronic devices including light emitting diodes(LEDs) because it exhibits a high thermal stability and a wide band-gapof 0.8 to 6.2 eV.

One of the reasons why GaN compound semiconductor has been highlightedis that it is possible to fabricate semiconductor layers capable ofemitting green, blue, and white light, using GaN in combination withother elements, for example, indium (In), aluminum (Al), etc.

Thus, it is possible to adjust the wavelength of light to be emitted, inaccordance with the characteristics of a specific apparatus, using GaNin combination with other appropriate elements. For example, it ispossible to fabricate a blue LED useful for optical recording or a whiteLED capable of replacing a glow lamp.

By virtue of the above-mentioned advantages of the GaN-based material,techniques associated with GaN-based electro-optic devices have rapidlydeveloped since the GaN-based LEDs became commercially available in1994.

The brightness or output of an LED manufactured using theabove-mentioned GaN-based material mainly depends on the structure of anactive layer, the extraction efficiency associated with externalextraction of light, the size of the LED chip, the kind and angle of amold used to assemble a lamp package, the fluorescent material used,etc.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a light emittingdevice and a method for manufacturing the same that substantiallyobviate one or more problems due to limitations and disadvantages of therelated art.

An object of the present invention is to provide a light emitting devicehaving a structure capable of achieving an enhancement in extractionefficiency while maintaining desired electrical characteristics when alight extracting structure is introduced into the light emitting device,and exhibiting an optimal extraction efficiency in cooperation with aphotonic crystal structure, and a method for manufacturing the lightemitting device.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein,alight emitting device comprises: a semiconductor layer; and a lightextracting layer arranged on the semiconductor layer and made of amaterial having a refractive index equal to or higher than a reflectiveindex of the semiconductor layer.

In another aspect of the present invention, a light emitting devicecomprises: a photonic crystal layer including at least two photoniccrystal structures arranged in the same plane on a semiconductor layer,the photonic crystal structures having different periodicities.

In another aspect of the present invention, a light emitting devicecomprises: a photonic crystal layer including a first photonic crystalhaving a periodic structure and a second photonic crystal having arandom structure, the first and second photonic crystals being arrangedin the same plane on a semiconductor layer.

In another aspect of the present invention, a light emitting devicecomprises: a reflective electrode; a semiconductor layer arranged on thereflective electrode, the semiconductor layer including a light emittinglayer; and a photonic crystal formed on the semiconductor layer, whereina distance between the reflective electrode and a center of the lightemitting layer is 0.65λ/n to 0.85λ/n, where “λ” represents a wavelengthof emitted light, and “n” represents a refractive index of thesemiconductor layer.

In another aspect of the present invention, a light emitting devicecomprises: a reflective electrode; a semiconductor layer arranged on thereflective electrode, the semiconductor layer including a light emittinglayer; and a photonic crystal formed on the semiconductor layer, whereina distance between the reflective electrode and a center of the lightemitting layer is an odd multiple of λ/4, where “λ” represents awavelength of emitted light, and “n” represents a refractive index ofthe semiconductor layer.

In still another aspect of the present invention, a method formanufacturing a light emitting device comprises: growing a plurality ofsemiconductor layers over a substrate; forming a first electrode on thesemiconductor layer; removing the substrate; forming a dielectric layerover the semiconductor layer exposed in accordance with the removal ofthe substrate; forming a plurality of holes in the dielectric layer;etching a surface of the dielectric layer formed with the holes, to forma plurality of grooves in the semiconductor layer; removing thedielectric layer; and forming a second electrode on a surface of thesemiconductor layer exposed in accordance with the removal of thedielectric layer.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a sectional view illustrating a structure for achieving anenhancement in the extraction efficiency of a light emitting device;

FIG. 2 is a graph depicting a variation in extraction efficiencydepending on an increase in the refractive index of a hemisphere in FIG.1;

FIG. 3 is a sectional view illustrating an embodiment of a horizontallight emitting device having a photonic crystal structure;

FIG. 4 is a graph depicting an extraction efficiency depending on aphotonic crystal depth in the structure of FIG. 3;

FIG. 5 is a graph depicting an extraction efficiency depending onetching depth in the case in which the thickness of a semiconductorlayer is limited;

FIG. 6 is a sectional view illustrating an embodiment of a verticallight emitting device having a photonic crystal structure;

FIG. 7 is a sectional view illustrating a vertical light emitting devicestructure for a computer simulation;

FIG. 8 is a sectional view illustrating the absorbance of a lightemitting layer in the structure of FIG. 7;

FIG. 9 is a photograph illustrating a radiation pattern exhibited in thecase in which the light emitting layer is sufficiently spaced apart froma mirror;

FIG. 10 is a graph depicting the results obtained by measuring anextraction efficiency while varying the periodicity of a photoniccrystal;

FIG. 11 is a graph depicting a variation in extraction efficiencydepending on the size of holes forming the photonic crystal;

FIG. 12 is a graph depicting a variation in extraction efficiencydepending on the etching depth of the photonic crystal;

FIG. 13 is a schematic view illustrating a principle that a photoniccrystal structure extracts light associated with a full-reflectionangle;

FIG. 14 is a sectional view illustrating a structure in which a photoniccrystal is introduced into the top layer of a horizontal light emittingdevice;

FIG. 15 is a sectional view illustrating a light emitting devicestructure formed on a patterned substrate;

FIG. 16 is a sectional view illustrating a light emitting devicestructure in which both the photonic crystal and the patterned substrateare introduced;

FIG. 17 is a graph depicting a variation in extraction efficiencydepending on the propagation distance of light in association with thephotonic crystal and the patterned substrate;

FIGS. 18 to 21 are photographs illustrating radiation patterns ofelectric dipoles;

FIG. 22 is a schematic view illustrating an arrangement of dipoles withrespect to the plane of a mirror;

FIG. 23 illustrates an extraction efficiency enhancement depending onthe gap between the mirror and the light emitting layer and anassociated radiation pattern, through a graph and photographs;

FIG. 24 is a photograph illustrating a radiation pattern in the case inwhich the gap between the light emitting layer and the mirror fallsunder a reinforced interference condition;

FIG. 25 is a graph depicting effects obtained in accordance withintroduction of a photonic crystal in a structure having a reinforcedinterference condition;

FIG. 26 is a graph depicting a variation in extraction efficiencydepending on the periodicity of the photonic crystal at a fixed etchingdepth;

FIG. 27 is a graph depicting a variation in extraction efficiencydepending on etching depth under a reinforced interference condition;

FIGS. 28 to 30 illustrate a first embodiment of the present invention,in which:

FIG. 28 is a sectional view illustrating the first embodiment of thepresent invention;

FIG. 29 is a sectional view illustrating a horizontal light emittingdevice according to the first embodiment of the present invention; and

FIG. 30 is a graph depicting an extraction efficiency depending on therefractive index of a light extracting layer;

FIGS. 31 to 35 illustrate a second embodiment of the present invention,in which:

FIG. 31 is a sectional view illustrating one example of a horizontallight emitting device structure according to the second embodiment ofthe present invention;

FIG. 32 is a sectional view illustrating another example of thehorizontal light emitting device structure according to the secondembodiment of the present invention;

FIG. 33 is a graph depicting a transmittance depending on the incidenceangles of a transparent conducting layer and a transparent metal layer;

FIG. 34 is a graph depicting an extraction efficiency depending on thethickness of the transparent conducting layer; and

FIG. 35 is a sectional view illustrating the light emitting deviceaccording to the second embodiment of the present invention;

FIG. 36 is a sectional view illustrating a third embodiment of thepresent invention;

FIGS. 37 to 47 illustrate a fourth embodiment of the present invention,in which:

FIG. 37 is a sectional view illustrating a light emitting device havingperiodicity-mixed photonic crystals;

FIG. 38 is a planar electro-microscopic photograph of the structureshown in FIG. 37;

FIG. 39 is a sectional electro-microscopic photograph of the structureshown in FIG. 37;

FIG. 40 is a plan view illustrating an example of the periodicity-mixedphotonic crystals;

FIG. 41 is a sectional view corresponding to FIG. 40;

FIG. 42 is a graph depicting an extraction efficiency of a structure inwhich periodicity-mixed photonic crystals are introduced;

FIGS. 43 to 46 are sectional views illustrating embodiments of theperiodicity-mixed photonic crystals; and

FIG. 47 is a sectional view illustrating a vertical light emittingdevice having periodicity-mixed photonic crystals;

FIGS. 48 to 53 illustrate a fifth embodiment of the present invention,in which:

FIG. 48 is a sectional view illustrating an example of a light emittingdevice;

FIG. 49 is a sectional view illustrating another example of a lightemitting device;

FIG. 50 is a graph depicting a variation in extraction efficiencydepending on the thickness of an ohmic electrode in the structure ofFIG. 49;

FIG. 51 is a graph depicting a variation in extraction efficiencydepending on the thickness of a p-type semiconductor layer in thestructure of FIG. 49;

FIG. 52 is a sectional view illustrating an example of a light emittingdevice package; and

FIG. 53 is a sectional view illustrating another example of the lightemitting device package;

FIGS. 54 to 66 illustrate a sixth embodiment of the present invention,in which:

FIG. 54 is a sectional view illustrating the step of forming an LEDstructure on a substrate;

FIG. 55 is a sectional view illustrating the step of removing thesubstrate, and forming a dielectric layer;

FIG. 56 is a sectional view illustrating the step of arranging a maskfor formation of a hole pattern in the dielectric layer;

FIG. 57 is a sectional view illustrating the step of forming a pluralityof holes in the dielectric layer;

FIGS. 58 to 62 are plan views illustrating examples of various holepatterns;

FIG. 63 is a schematic view illustrating a dry etching process;

FIG. 64 is a sectional view illustrating the step of forming a photoniccrystal in an n-type semiconductor layer;

FIG. 65 is scanning electron microscope (SEM) image of the photoniccrystal structure; and

FIG. 66 is a sectional view illustrating the light emitting devicestructure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown.

The present invention may, however, be embodied in many alternate formsand should not be construed as limited to the embodiments set forthherein. Accordingly, while the present invention is susceptible tovarious modifications and alternative forms, specific embodimentsthereof are shown by way of example in the drawings and will herein bedescribed in detail. It should be understood, however, that there is nointent to limit the invention to the particular forms disclosed, but onthe contrary, the invention is to cover all modifications, equivalents,and alternatives falling within the spirit and scope of the invention asdefined by the claims.

Like numbers refer to like elements throughout the description of thefigures. In the drawings, the thickness of layers and regions areexaggerated for clarity.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. It will also be understood that if part of an element, such asa surface, is referred to as “inner,” it is farther to the outside ofthe device than other parts of the element.

In addition, relative terms, such as “beneath” and “overlies”, may beused herein to describe one layer's or region's relationship to anotherlayer or region as illustrated in the figures.

It will be understood that these terms are intended to encompassdifferent orientations of the device in addition to the orientationdepicted in the figures. Finally, the term “directly” means that thereare no intervening elements. As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, components, regions, layersand/or sections, these elements, components, regions, layers and/orsections should not be limited by these terms.

These terms are only used to distinguish one region, layer or sectionfrom another region, layer or section. Thus, a first region, layer orsection discussed below could be termed a second region, layer orsection, and similarly, a second region, layer or section may be termeda first region, layer or section without departing from the teachings ofthe present invention.

The extraction efficiency of a semiconductor light emitting device isdetermined by the refractive index difference between a semiconductorlight emitting layer of the semiconductor light emitting device, fromwhich light is emitted, and a medium (air or epoxy resin) through whichthe emitted light is finally observed. The extraction efficiency of asemiconductor medium is only several percentages because thesemiconductor medium typically has a high refractive index (n>2).

For example, in the case of a blue light emitting device made of agallium nitride (n=2.4), the extraction efficiency for light emittedthrough a top layer of the light emitting device is only about 9% whenit is assumed that the external medium is epoxy resin (n=1.4). Lightother than the light emitted through the top layer is confined in theinterior of the device due to a full reflection procedure, and thendisappears as it is absorbed into an absorption layer such as a quantumwell layer.

In order to enhance the extraction efficiency of such a semiconductorlight emitting device, it is necessary to change the structure of thedevice such that light subjected to a full reflection procedure can beexternally extracted. The simplest scheme for changing the structure ofthe semiconductor light emitting device is to coat a hemisphere made ofa material having a high refractive index over the top layer of thedevice.

Since the incidence angle of light incident to a light incidence surfacecorresponds to an angle defined between the incident light and the lightincidence surface, the incidence angle of light incident to thehemisphere is always 90° at any point on the hemisphere. Thetransmittance of light between two mediums having different refractiveindexes is highest when the light incidence angle is 90°. Also, there isno angle, at which full reflection occurs, in any direction.

Practically, in the case of a semiconductor light emitting device, ahemisphere made of epoxy resin is coated over the device. In this case,the hemisphere contributes to protecting the surface of the device andenhancing the extraction efficiency of the device.

In FIG. 1, a method for more effectively obtaining the above-describedeffects is illustrated. The illustrated method is additionalintroduction of a hemisphere 3 having a refractive index similar to thatof a semiconductor between an epoxy layer 1 and a semiconductor device2. In this case, as the refractive index of the additionally-introducedhemisphere 3 approximates to the refractive index of a semiconductor,the extraction efficiency of the semiconductor device 2 increases, asshown in FIG. 2.

This is because the critical angle between the semiconductor device 2and the additionally-introduced hemisphere 3 increases.

As an example of a material exhibiting no absorption of light in avisible range and being transparent, TiO₂ may be proposed. For example,when it is assumed that a hemisphere made of the above-describedmaterial is applied to a red light emitting device, an enhancement inextraction efficiency corresponding to about 3 times the extractionefficiency of conventional cases can be theoretically obtained.

The additional introduction of a hemisphere having a high refractiveindex is a very simple and effective method. In order to apply thismethod, however, it is necessary to use a transparent material having ahigh refractive index and exhibiting no absorption of light in a lightemission wavelength range.

Furthermore, there may be difficulties in association with processes formanufacturing a hemisphere having a size sufficiently covering the lightemitting device and attaching the manufactured hemisphere to the lightemitting device.

Another method for achieving an enhancement in external-extractionefficiency is to deform the side surfaces of a light emission structureinto a pyramidal shape. This method utilizes the principle that lighttraveling laterally while being fully reflected within a light emittingdevice emerges from the top layer of the device after being reflectedfrom the pyramidal surfaces.

However, this method has a drawback in that a desired enhancement isreduced as the size of the device increases. This is caused byabsorption loss inevitably involved in the lateral travel of light. Forthis reason, in order to obtain an increased enhancement in a practicalstructure, it is important to cause light to be externally emitted aftertraveling along a path as short as possible.

To this end, research has been conducted to introduce a structurecapable of alleviating the conditions of full reflection into a lightemitting device. Representatively, there is a method in which thestructure of a light emitting device is designed to have a resonatorstructure, to induce an output in a specific direction, starting from aninitial stage, or a method in which hemispherical lenses having a sizeof several microns or more are arranged on the top layer of a lightemitting device, to achieve an increase in critical angle.

However, the above-mentioned methods have not been practically used dueto difficulties associated with manufacture processes. There is anothermethod in which a rough surface having a size substantiallycorresponding to the wavelength of light is introduced into the outputportion of a light emitting device, to achieve an enhancement inextraction efficiency through a light scattering procedure.

For the method for forming a rough surface on the top layer of a lightemitting device, various chemical processes have been developed inassociation with materials used to manufacture light emitting devices.When light reaches a rough surface, a part of the light can pass throughthe rough surface even at an incidence angle corresponding to fullreflection.

However, the light transmittance obtained by one scattering procedure isnot so high. Accordingly, in order to expect a high light extractioneffect, the same scattering procedure should be repeatedly carried out.For this reason, in the case of a light emitting device containing amaterial having a high absorbance, the extraction efficiency enhancementcaused by the rough surface is small.

As compared to the above-mentioned method, when a photonic crystalhaving a spatially-periodic refractive index arrangement is introduced,it is possible to greatly enhance the extraction efficiency. Also, whenan appropriate photonic crystal periodicity is selected, it is possibleto adjust the directionality of the output of the light emitting device.Since view angle depends on the application of the light emittingdevice, it is considered that the design of the directionality meetingthe application is an important task.

A large-area photonic crystal structure can be realized using holographylithography, ultraviolet (UV) photolithography, nano-imprintedlithography, or the like. Accordingly, this technique can be easilyimplemented for practical use thereof.

Efforts to enhance the extraction efficiency of a light emitting devicethrough photonic crystals were made, starting from research for theadjustment of spontaneous emission rate using a photonic crystals.

Thereafter, the fact that photonic crystals contribute to an enhancementin the extraction efficiency of a light emitting device wastheoretically proved. The contribution procedure of photonic crystals tothe extraction efficiency enhancement is mainly summarized into twoprinciples.

One principle is to cut off movement of light in a plane direction usinga photonic band-gap effect, and thus, to extract light in a verticaldirection. The other principle is to couple light with a mode having ahigh state density arranged outside a light cone in a dispersion curve,and thus, to externally extract the light.

The above-mentioned two principles can be independently applied inaccordance with the periodicity of photonic crystals. However, it ispossible to well define the photonic band-gap effect or the statedensity of the dispersion curve only when photonic crystals are formedunder the condition in which there is a thin film which has a thicknesscorresponding to a half wavelength and a high refractive index contrastin a vertical direction.

Furthermore, since hole structures forming a photonic crystal extendthrough the light emitting layer, loss of a gain medium is inevitablygenerated. Additionally, it is impossible to avoid a reduction ininternal quantum efficiency caused by surface nonradiativerecombination.

It is considered that the photonic band-gap mirror effect or strongdispersion characteristics are applicable to specific cases because itis difficult to realize them in the structure of a general lightemitting device. In order to solve this problem, a photonic crystalshould be formed only on the surface of the light emitting device,without being formed on a positive medium of the light emitting device.

In this case, although it is impossible to utilize strong dispersioncharacteristics as in the case in which a photonic crystal is introducedinto a thin film, it is possible to externally extract light associatedwith full reflection by coupling the light with a periodic structure inaccordance with a general diffraction theory.

Currently, efforts to enhance the extraction efficiency withoutdegrading the characteristics of a light emitting layer made ofsemiconductor by spatially separating a photonic crystal from the lightemitting layer are being actively made.

Also, there was an effort to achieve an enhancement in extractionefficiency for a light emitting device using an InGaAs quantum well, inaccordance with the same method as mentioned above. In addition, inassociation with an organic light emitting device, there was a reportthat it is possible to enhance the extraction efficiency of the organiclight emitting device by 1.5 times, using a photonic crystal formed on aglass substrate.

There was also a method, in which a periodic photonic crystal structureis introduced into a semiconductor surface, to extract light confineddue to full reflection, through a diffraction procedure, as mentionedabove. For example, there was a report that it is possible to achieve anenhancement in extraction efficiency by forming a photonic crystalhaving a periodicity of 200 nm on a p-type GaN semiconductor surface.

In addition, there was a method in which a photonic crystal is formedeven on the positive medium region of a GaN-based light emitting device,to achieve an increased enhancement in extraction efficiency using aphotonic band-gap effect. In this method, however, there is a drawbackof a reduction in extraction efficiency when input current increases.This is because, when a photonic crystal is introduced under thecondition in which even the light emitting layer is etched, thecurrent-voltage characteristics are particularly degraded, as mentionedabove.

As apparent from the above description, the principle of an enhancementin external extraction efficiency of a light emitting device can besummarized into a method in which the structure of a light emittingdevice is changed to alleviate the conditions of full reflection, amethod in which a rough surface is introduced into the surface of alight emitting device, a method in which a photonic crystal is formed ina thin film having a high refractive index contrast, to utilize aphotonic band-gap effect, and a method in which a photonic crystal isseparated from a light emitting layer, to externally extract lightconfined due to full reflection, through a diffraction procedure.

Of these methods, it is considered that the method, in which a periodicphotonic crystal structure is introduced into the surface of a lightemitting device, to achieve an enhancement in extraction efficiency, isbest, taking into consideration the structure reality and efficiencyenhancement of the light emitting device.

FIG. 3 illustrates a horizontal GaN-based light emitting device in whicha GaN semiconductor layer 20 is grown over a substrate 10 made ofsapphire having a refractive index (n=1.76) lower than GaN. Since theGaN semiconductor layer 20 has a total thickness reaching about 5 μm, itis considered to be a waveguide structure in which various higher modesare present. The GaN semiconductor layer 20 includes, as a top layerportion thereof, a p-type GaN semiconductor layer 21. A multi-quantumwell layer is arranged beneath the p-type GaN semiconductor layer 21, asa light emitting layer 22.

An n-type GaN semiconductor layer 23 is arranged beneath the lightemitting layer 22. A buffer layer 24 may be interposed between then-type GaN semiconductor layer 23 and the substrate 10. Also, areflective film (not shown) may be formed on the substrate 10, oppositeto the GaN semiconductor layer 20.

In order to uniformly supply current over the overall surface in thehorizontal GaN-based light emitting device, a transparent electrodelayer 30, which is typically made of indium tin oxide (ITO), isdeposited over the p-type GaN semiconductor layer 21. Accordingly, themaximum etchable range for the introduction of a photonic crystal 40into the horizontal GaN-based light emitting device corresponds to thesum of the thickness of the transparent electrode layer 30 and thethickness of the GaN semiconductor layer 21. Generally, the transparentelectrode 30 and p-type GaN semiconductor layer 21 have a thickness of100 to 300 nm.

A calculation through a computer simulation (three-dimensionalfinite-difference time-domain (3D FDTD)) was conducted to identify theaspect of a variation in extraction efficiency depending on the etchingdepth of the photonic crystal. The results of the calculation throughthe computer simulation are depicted in FIG. 4. Referring to FIG. 4, itcan be seen that there are two particular phenomena.

First, the extraction efficiency increases gently in proportion to theetching depth of the photonic crystal 40, and then increases sharply,starting from a region where the GaN semiconductor layer 20 begins to beetched. Second, at a certain etching depth or more, there is a tendencythat the extraction efficiency is saturated without further increasing.

Collectively taking into consideration the above-mentioned twophenomena, it can be understood that it may be necessary to introducethe photonic crystal 40 into a region including the GaN semiconductorlayer 20 under the condition in which the photonic crystal 40 has acertain etching depth or more.

Since the etching depth, at which the extraction efficiency issaturated, is similar to the thickness of the general p-type GaNsemiconductor layer 21, it is possible to obtain a theoreticalextraction efficiency saturation value without causing the region of thelight emitting layer 22 to be etched.

However, in accordance with a practical experiment for checking avariation in light output depending on the etching depth of the photoniccrystal 40, it is found that, when the p-type GaN semiconductor layer 21is etched to a specific depth or more, the light output is ratherreduced.

The reason why the light output is reduced in spite of the fact that thelight emitting layer 22, namely, the quantum well structure, is notexposed is that an increase in resistance occurs due to a reduction inthe volume of the p-type GaN semiconductor layer 21. Such a resistanceincrease may be more serious in application fields requiring high-powerlight emitting devices.

That is, the structure of a currently-available light emitting devicewith a photonic crystal cannot utilize the etching depth, at which amaximum extraction efficiency is obtained, due to an increase inresistance, from an optical point of view. Therefore, the matter to besolved is to develop a new structure to which an etching depth, at whichthe extraction efficiency obtained by the photonic crystal is maximum,is applicable without causing an increase in resistance.

Referring to the graph depicting a variation in extraction efficiencydepending on etching depth, which is induced through the above-mentionedcomputer simulation, a clue to a new structure can be obtained. Oneparticular phenomenon found in the graph is that, when the etching depthof the photonic crystal 40 transits from the ITO transparent electrodelayer 30 to the GaN semiconductor layer 20, the extraction efficiencyincreases remarkably.

Basically, this is because the refractive index of ITO (n=1.8) is lowerthan the refractive index of the GaN semiconductor layer 20. Theintroduction of the photonic crystal 40 into a region having a lowrefractive index cannot be a great help to an enhancement in extractionefficiency. This can be understood that the photonic crystal 40functions to extract light subjected to a full reflection procedure.

The extraction efficiency depends on how much the photonic crystalrecognizes the region of the photonic crystal 40. Generally, when a fullreflection phenomenon occurs, a surface evanescent wave is generatedbetween two surfaces having different refractive indexes. The surfaceevanescent wave is present along the boundary surface of the twosurfaces, and has characteristics of a exponential reduction inintensity in a direction perpendicular to the boundary surface.

For example, when only the ITO transparent electrode layer 30 is etched,the light associated with full reflection can recognize the photoniccrystal 40 formed in the ITO transparent electrode layer 30 to a levelcorresponding to the intensity of the surface evanescent wave.Accordingly, it is impossible to expect a great enhancement inextraction efficiency.

Consequently, in order to extract a larger amount of light through thephotonic crystal 40, a strong interaction between the photonic crystal40 and the light is required. From a physical point of view, spatialoverlap between the full reflection mode formed in the GaN-based lightemitting device and the structure of the photonic crystal 40 functionsas an important factor. That is, in order to disturb the surfaceevanescent wave generated during the full reflection procedure, it isnecessary to etch the GaN semiconductor layer 20, for formation of thephotonic crystal 40.

Meanwhile, the etching of the GaN semiconductor layer may also mean thata material having a refractive index similar to or higher than that ofthe GaN semiconductor layer is etched. That is, when the formation ofthe photonic crystal 40 is carried out after a material having arefractive index similar to that of the p-type GaN semiconductor layeris deposited over the p-type GaN semiconductor layer, it is possible toexpect effects similar to those of the above-described case, even thoughthe p-type GaN semiconductor layer is not etched.

Also, when the newly-deposited material has a refractive index higherthan that of GaN, properties superior to the above-described effects ofthe photonic crystal may be exhibited. This is because the effects ofthe photonic crystal depend on the refractive index difference betweentwo materials basically forming the light extracting structure.

Accordingly, when a light extracting structure for an enhancement inextraction efficiency is introduced into a GaN-based light emittingdevice, it is possible to achieve an enhancement in extractionefficiency while maintaining desired electrical characteristics byforming a light extracting structure such as photonic crystal in amaterial additionally introduced over the top layer of the lightemitting device, namely, the GaN semiconductor layer (typically, p-GaN),without etching the GaN semiconductor layer.

The maximum etchable range for the introduction of the photonic crystal40 into a horizontal GaN-based light emitting device is limited by thethickness (100 to 300 nm) of the p-type GaN semiconductor layer 21forming the top layer of the light emitting device. For this reason,there may be a limitation on extraction efficiency.

FIG. 6 illustrates an example of a vertical light emitting device. Theillustrated vertical light emitting device has a structure in which thematerial of a substrate, namely, sapphire, is removed during a growthprocedure of a GaN semiconductor layer 20, in accordance with a laserabsorption process, and a reflective ohmic electrode 50 is formed over ap-type GaN semiconductor layer 21, using a multi-layer metal thin filmmade of Ni, Ag, etc. to function as both a mirror and an electrode.

The vertical GaN-based light emitting device is different from a generalhorizontal GaN-based light emitting device in that the flow direction ofcurrent is vertical due to the removal of an insulator, namely,sapphire, and the light output surface in the case of FIG. 6 is reversedsuch that light is output from an n-type GaN semiconductor layer 23.

The fact that current flows in a vertical direction in the verticallight emitting device structure means that it is quite probable that thesupplied current can reach a light emitting layer 22, namely, a quantumwell layer. This means that an enhancement in internal quantumefficiency can be achieved.

Also, the vertical light emitting device structure has characteristicscapable of easily discharging heat because sapphire, which is aninsulator, is removed, and a conductor is formed over the p-type GaNsemiconductor layer 21. These characteristics may provide advantagesupon designing a high-power light emitting device.

Practically, when the amount of the supplied current is larger thanseveral hundred mA in the case of a general GaN-based blue lightemitting device, the output is rather reduced. This may be analyzed tobe caused by a degradation in the internal quantum efficiency of thequantum well due to an increase in the internal temperature of thedevice caused by the low heat conductivity of the sapphire substrate.

In addition to the physical characteristics of easy current flow andeasy heat discharge, the vertical blue light emitting device structurehas optical characteristics worth being taken into consideration inassociation with an enhancement in extraction efficiency. This will bedescribed hereinafter.

First, the vertical light emitting device structure has an advantage inthat, since the top layer of the vertical light emitting devicestructure is constituted by the n-type GaN semiconductor layer 23, aphotonic layer 60 can be introduced into the n-type GaN semiconductorlayer 23 which is relatively thicker than the p-type GaN semiconductorlayer 21. Generally, the extraction efficiency enhancing effect obtainedthrough the photonic crystal 60 is proportional to the etching depthuntil the enhancement efficiency is saturated.

Accordingly, it is possible to form a photonic crystal structure havinga desired depth without a problem involved in the introduction of thephotonic crystal, namely, an increase in resistance caused by theetching of the p-type GaN semiconductor layer, or any limitationassociated with a surface nonradiative recombination caused by an activelayer, namely, a quantum well layer. Also, since the periodicity forproviding a maximum extraction efficiency varies slightly depending onthe etching depth, it is possible to utilize structural conditionsallowed by the given etching technique.

Also, in the vertical light emitting device, a quantum well layer (lightemitting layer 22), which is a light emitting region, and a mirror(reflective ohmic electrode 50) are arranged at positions shorter thanthe wavelength of emitted light.

That is, as described above, in the structure of the vertical lightemitting device, a reflective ohmic electrode 50 functioning as both amirror and an electrode is formed over the p-type GaN semiconductorlayer 21. Accordingly, the thickness of the p-type GaN semiconductorlayer 21 corresponds to the gap between the light emitting layer 22 anda metal mirror in the light emitting device.

Generally, when a mirror having a high reflectance is present at aposition near the light emitting layer 22, the light emissionperformance of the light emitting device may greatly vary, as comparedto the case in which there is no mirror. That is, a variation in decayrate may occur in accordance with the gap between the light emittinglayer 22 and the mirror. It may also be possible to adjust a radiationpattern in accordance with the gap between the light emitting layer 22and the mirror. When these characteristics are appropriately used, it ispossible to greatly enhance the extraction efficiency of the lightemitting device.

Hereinafter, the procedure for determining structural factors of aphotonic crystal applicable to a vertical GaN-based light emittingdevice and calculating a relative ratio of extraction efficiencyenhancements obtainable by respective structural factors will bedescribed.

The total efficiency of the vertical light emitting device structurecorresponds to the efficiency obtained in accordance with verticalradiation because there is no radiation through side surfaces of thesubstrate in the vertical light emitting device structure, as comparedto the horizontal structure. In FIG. 7, an example of a light emittingdevice structure to be analyzed through a computer simulation isillustrated. As shown in FIG. 7, the light emitting device structureincludes a light-emitting semiconductor layer 80 formed with a photoniccrystal 60. An epoxy resin layer 70 having a refractive index of 1.4,which is usable as a sealant, is arranged outside the photonic crystal60.

It is impossible to completely include the size of a general lightemitting device in the calculation structure due to a limited computermemory. In order to solve this problem, a method, in which perfectmirrors (not shown) are arranged at opposite ends of a light emittingdevice structure having a finite size (12 μm) was used.

Also, an absorbance k of 0.045 was given to the inside of the lightemitting layer (quantum well layer 22) of the light emitting device 80,as shown in FIG. 8. However, for the convenience of analysis, an actualmetal mirror, which exhibits a certain absorbance and is arranged at alower end of the structure, was replaced by a perfect mirror having areflectance of 100%.

Since an interference effect by a mirror should always be taken in thevertical structure, the relative position of the light emitting layer 22to the mirror in the structure is an important parameter. This isbecause, when the radiation pattern is changed due to an interferenceeffect generated between the mirror and the light emitting layer 22, thestructural factors of the photonic crystal 60 to act effectively may bevaried. That is, it may be considered that the angle of light, at whichefficient light extraction occurs in accordance with a diffractionprocedure, depends on the periodicity of the photonic crystal 60.

In this case, it is intended to calculate only the effect obtained bythe photonic crystal 60 under the condition in which mirror effects areexcluded. In order to exclude interference effects caused by the mirror,the gap between the mirror and the light emitting layer 22 is set to belong or to correspond to a middle condition between a reinforcedinterference condition and an offset interference condition.

The radiation pattern obtained when the light emitting layer 22 is freeof the interference effects of the mirror, as described above, isillustrated in FIG. 9. This radiation pattern may be considered to be aspherical wave, even though fine interference patterns are stillexhibited at specific angles.

A variation in extraction efficiency depending on the periodicity of thephotonic crystal 60 is illustrated in FIG. 10. As shown in FIG. 10, theperiodicity “a” of the photonic crystal 60, at which a maximumextraction efficiency can be obtained, is about 800 nm, and the relativeenhancement in extraction efficiency is about 2 times. In this case, theetching depth was set to 225 nm, and the radius of holes 61 forming thephotonic crystal 60 was set to 0.25a when “a” represents the periodicityof the photonic crystal 60.

A variation in extraction efficiency depending on the size of the holes61 forming the photonic crystal 60 is illustrated in FIG. 11. In thiscase, the etching depth was set to 225 nm, and a periodicity of 800 nmwas selected. Referring to FIG. 11, it can be seen that, when the sizeof the holes 61 forming the photonic crystal 60 corresponds to 0.35a, amaximum extraction efficiency is obtained, and the relative enhancementincreases to 2.4 times.

As described above, the vertical GaN-based light emitting device isadvantageous in that the limitation on etching depth is small. Althoughthe maximum etching depth in the horizontal structure is determined bythe thickness of the p-type GaN semiconductor layer (actually, abouthalf of the thickness of the p-GaN layer when an increase in resistanceis taken into consideration), it is possible to use the thickness of then-type GaN semiconductor layer (about 3 μm), which is much larger thanthe thickness of the p-type GaN semiconductor layer, in the verticalstructure.

In order to utilize the above-described advantage of the verticalstructure, an optimal periodicity depending on the etching depth forformation of the photonic crystal was checked while sequentially varyingthe etching depth.

As mentioned above in association with the research for the horizontalstructure, there is a tendency that the extraction efficiency issaturated, at a certain etching depth or more.

However, there is an interesting fact that, when the etching depthincreases, the extraction efficiency obtained by a photonic crystalstructure having a long periodicity increases continuously. This isworthy of notice in that it is possible to use a photonic crystalstructure having a long periodicity, which can be easily technicallyimplemented, while increasing the etching depth.

The reason why the extraction efficiency of the photonic crystalstructure having a long periodicity increases continuously as theetching depth increases may be analyzed as follows (FIG. 13).

First, light can pass through two mediums having different refractiveindexes, only when the phase-matching condition in a plane direction issatisfied.

Second, when light propagates from a medium having a high refractiveindex to a medium having a low refractive index, it is impossible tosatisfy the phase-matching condition at a specific angle or more. Thisspecific angle is called a “critical angle”. Full reflection occurs atthe critical angle or more.

Third, the photonic crystal assists light associated with an anglecausing full reflection to be externally extracted. That is, when thephotonic crystal is coupled with light, the quantity of motion of thephotonic crystal increases to allow the light associated with fullreflection to satisfy the phase-matching condition.

Fourth, the quantity of motion of the photonic crystal is inverselyproportional to the periodicity of the photonic crystal. That is, sincethe photonic crystal, which has a short periodicity, can generate alarge quantity of motion, it can effectively extract light, which isincluded in light associated with full reflection, but propagates in adirection close to a horizontal direction far way from the criticalangle. On the other hand, the photonic crystal, which has a longperiodicity, is effective in extracting light propagating in a directionclose to a vertical direction.

Fifth, in accordance with a theory of wave optics, the full reflectionprocedure carried out in a waveguide structure can be explained inassociation with modes. For example, light having an incidence angleclose to a horizontal direction corresponds to a basic waveguide mode,and light having an incidence angle closer to a vertical directioncorresponds to a higher waveguide mode.

Sixth, the GaN-based light emitting device can also be considered as awaveguide structure having a thickness of several microns or more.

Accordingly, it can be seen that the photonic crystal having a shortperiodicity is suitable to extract the basic wavelength mode, whereasthe photonic crystal having a long periodicity is suitable to extract ahigher wavelength mode. Thus, it is possible to determine an appropriatephotonic crystal applicable to a GaN-based light emitting device, takinginto consideration the above-described facts.

Generally, the basic wavelength mode exhibits a tendency of a saturationin extraction efficiency at a certain photonic crystal etching depth(˜λ/n) or more. On the other hand, the higher wavelength mode exhibits atendency of a continuous increase in extraction efficiency at anincreased photonic crystal etching depth.

Consequently, the extraction efficiency for a higher waveguide mode bythe photonic crystal structure having a long periodicity increasescontinuously as the etching depth increases.

In order to obtain a maximum extraction efficiency, a task foroptimizing the structural factors of the photonic crystal was conductedthrough a calculation using a computer simulation. As a result, it wasfound that the extraction efficiency has a close relation with etchingdepth, hole size, periodicity, etc.

In particular, in the case of a vertical GaN-based light emittingdevice, there is no limitation on etching depth because arelatively-thick n-type GaN semiconductor layer is used to form aphotonic crystal. Accordingly, when a large etching depth is introduced,it is also possible to increase the possibility that the periodicity,which can be implemented using the current technique, is selectable.

As described above, the incidence angle of effectively-acting lightvaries depending on the periodicity of the photonic crystal. That is, ina photonic crystal having only one periodicity, there is an incidenceangle range in which a relatively-low diffraction efficiency isexhibited.

However, in order to maximize the extraction efficiency, the photoniccrystal should exhibit a high diffraction efficiency for angles largerthan the critical angle. Accordingly, in the case of a photonic crystalstructure having two or more mixed periodicities, superior extractionefficiency characteristics may be exhibited, as compared to a photoniccrystal in which only one periodicity is independently present.

A similar principle may be applied to a horizontal GaN-based lightemitting device. The method for introducing a photonic crystal into ahorizontal GaN-based light emitting device to achieve an enhancement inexternal extraction efficiency may be mainly divided into two methods inaccordance with the introduction position of the photonic crystal.

One method is illustrated in FIG. 14. This method is to etch a specificportion of a semiconductor layer 20 formed over a sapphire substrate 10,namely, a p-type GaN semiconductor layer 21 arranged at the top of thelight emitting device, as shown in FIG. 14. Where a transparentconductive layer 30 is formed over the p-type semiconductor layer 21,the transparent conductive layer 30 is also etched.

The other method is illustrated in FIG. 15. This method is to grow a GaNsemiconductor layer 20 over a patterned sapphire substrate (PSS) 12previously formed with a pattern 11, as shown in FIG. 15.

Meanwhile, a method to apply both the above-described structures may beimplemented, as shown in FIG. 16. Graphs for comparison of extractionefficiencies respectively obtained in the above-described structures areshown in FIG. 17.

In the graphs, the horizontal axis represents the propagation distanceof light in a calculation space, and the vertical axis represents theamount of externally-extracted light depending on the propagationdistance of the light.

Referring to the graphs, it can be seen that, in the horizontalstructure (reference), to which no periodic structure is applied, asaturation in extraction efficiency occurs before the propagationdistance of light reaches 10 μm. Since the horizontal structure canextract only the light within the critical angle, most light exits inone transmission (reflection) procedure.

On the other hand, in the case, to which the structure of the photoniccrystal 40 or the patterned sapphire substrate 12 with the pattern 11 isapplied, a continuous increase in extraction efficiency is exhibiteduntil the propagation distance of light reaches 100 μm. This is becauselight associated with full reflection is externally extracted inaccordance with a diffraction procedure every time the light meets theperiodic structure.

The extraction efficiency is finally saturated due to the absorbance ofthe inner material of the device. Accordingly, the basic principleapplied to achieve an enhancement in external extraction efficiency inaccordance with change of the structure or introduction of a periodicstructure is to externally extract light within a propagation distanceas short as possible such that the light is subjected to a reducedabsorption loss.

From the above results, it can be seen that, when both the structure ofthe photonic crystal 40 and the patterned sapphire substrate 12 with thepattern 11 are applied, a maximum extraction efficiency is obtained.

The structure of the horizontal GaN-based light emitting device, inwhich the GaN semiconductor layer 20 is grown over the patternedsapphire substrate 12 formed with the pattern 11, and the photoniccrystal 40 is applied to the top of the GaN semiconductor layer 20, maybe technically defined as a structure in which photonic crystals havingdifferent periodicities are independently applied to different planes.

It is impossible to apply a photonic crystal structure having mixedperiodicities to a vertical GaN-based light emitting device, differentfrom the horizontal light emitting device. This is because the verticalGaN-based light emitting device has a substrate-removed structure.However, it is possible to apply a photonic crystal structure havingdifferent periodicities within one plane by utilizing the advantage ofno limitation on etching depth.

Generally, it is possible to adjust the characteristics of the lightemitting layer 22 when the gap between the light emitting layer 22 and ametal mirror having a high reflectance is shorter than the wavelength oflight generated from the light emitting layer 22.

FIGS. 18 to 22 depict which phenomenon does occur when an electricdipole, which generates light, is arranged at a position very close to aperfect mirror, through an FDTD computer simulation. The electric dipolemeans an electron vibrating in a specific direction in accordance withpolarized light.

In accordance with an antenna theory, light generated from an electricdipole has a radiation pattern having peaks distributed in a directionperpendicular to a vibrating direction of an electron. That is, when anelectric dipole is positioned in a single dielectric space where nohighly-reflective mirror is present, light generated from the electricdipole has a radiation pattern having peaks distributed in a directionperpendicular to each polarization direction.

However, when a mirror having a high reflectance is arranged near theelectric dipole at a distance shorter than the wavelength of light, agreat variation in light emission characteristics occurs. In accordancewith the gap between the electric dipole and the mirror, lightconcentrates around a vertical line sometimes, or propagates along theplane of the mirror sometimes.

Taking such phenomena into consideration, it is possible to achieve anenhancement in extraction efficiency by initially applying the conditionfor generating light mainly having vertical components from the quantumwell layer, namely, the light emitting layer 22. Also, although notdepicted in FIGS. 18 to 21, it is possible to adjust a constant ofnature, t (Decay rate corresponds to a reciprocal number of “t”),namely, the time taken for an excited electron to transit to a groundstate.

As apparent from the above description, the feature that the lightemission characteristics of the light emitting layer 22 can be adjustedmainly means the following two features.

One feature is that the output radiation pattern can be adjusted usingan interference effect generated between light generated from the lightemitting layer 22 and light reflected from the metal mirror. The otherfeature is that the decay rate can be adjusted through an interactionbetween a dipole in the light emitting layer 22 and an image dipolegenerated by the metal mirror.

The first feature can be explained using a traditional interferencephenomenon of light. When the gap between the mirror and the lightemitting layer 22 is sufficiently long to ignore an interference effectcaused by the mirror, the light generated from the light emitting layer22 can be considered as a spherical wave having a constant coefficientin all directions.

In the case in which the mirror is positioned near the light emittinglayer 22 such that the radiation pattern is adjustable, occurrence ofreinforced interference is advantageous in terms of extractionefficiency.

Again referring to the light emitting device as shown in FIG. 6, the gapbetween the light emitting layer 22 and the mirror (reflective electrodeor reflective ohmic electrode 50) in the vertical light emitting devicestructure corresponds to the thickness of the p-type GaN semiconductorlayer 21. Accordingly, it is necessary to select an appropriatethickness of the p-type GaN semiconductor layer forming a verticalradiation pattern, within a range causing a degradation in electricalcharacteristics.

The second feature associated with the adjustment of light emissioncharacteristics has a close relation with fields of resonator quantumelectrodynamics. However, this feature, namely, the principle ofqualitatively adjusting a decay rate, can be easily explained, using thesymmetry of the mirror.

FIG. 22 schematically shows the figures of vertically andhorizontally-polarized electric dipoles arranged around the plane of amirror 51. In accordance with an electromagnetic field theory, theelectric field on the plane of the mirror 51 should always be “0”.

Using this principle, the situation that electric dipoles are arrangedaround the mirror 51 can be realized through a combination of anelectric dipole and an image dipole arranged at opposite sides of themirror 51 while being spaced apart from the mirror 51 by the samedistance.

For example, in the case of an electric dipole having polarities in az-direction, the dipole moment thereof should have the same direction asthat of an image dipole, in order to satisfy the electric fieldcondition on the plane of the mirror 51. Accordingly, as the gap betweenthe electric dipole and the mirror 51 is reduced, an effect as if twoelectric dipoles overlap with each other is generated. As a result, aneffect of increasing the decay rate by four times is generated.

On the other hand, when an electric dipole having polarities in ahorizontal direction is applied to the electric field condition on theplane of the mirror 51, an image dipole with a dipole moment in adirection opposite to that of the electric dipole is always induced.Accordingly, as the horizontally-polarized electric dipole approachesthe plane of the mirror 51, the decay rate approximates to “0”.

An arithmetic calculation was conducted, through an FDTD computersimulation, for a variation in extraction efficiency enhancement and avariation in the decay rate of the light emitting layer depending on avariation in output pattern, while adjusting the gap between the mirrorand the light emitting layer, as shown in FIG. 23. In this case, themirror was assumed as a perfect mirror having a reflectance of 100%, andthe thickness of the light emitting layer was set to 12.5 mm.

First referring to the results as to extraction efficiency enhancement,it can be seen that an extraction efficiency peak and an extractionefficiency valley are exhibited at intervals of about ¼ wavelength oflight. This is an evidence identifying that the radiation pattern isvaried due to an interference effect of light, and thus, the extractionefficiency is adjusted.

After observing the radiation patterns at the peak and valley, it can beseen that, actually, strong emission of light in a vertical directionoccurs at the peak, whereas, at the valley, there is no or littlevertical light, and most light is emitted in a state of being inclinedat a specific angle larger than the critical angle.

It can be seen that a maximum extraction efficiency is obtained when thegap between the light emitting layer and the mirror is about ¾(λ/n), anda large extraction efficiency is obtained when the gap corresponds toabout an odd multiple of λ/4n.

In order to practically apply the interference effect obtained by themirror to the vertical GaN-based light emitting device structure, it isnecessary to solve the matters assumed in the computer simulation. Inparticular, although the light emitting layer was assumed approximatelyas a point light source, in the computer simulation, the quantum welllayer of the practical light emitting device has a thickness of about 50to 100 nm in accordance with the number of laminated pairs thereof.

However, when the thickness of the light emitting layer is larger thanλ/4n, the interference effect by the mirror is reduced, and may finallydisappear. Accordingly, it is necessary to provide a growth techniquefor reducing the thickness of the quantum well layer while maintaining adesired internal quantum efficiency.

Next, referring to the results as to decay rate variation, it can beseen that there are characteristics of an increase in decay rate as thegap between the mirror and the light emitting layer is reduced. That is,as the mirror approaches the light emitting layer, the circulationprocedure of a gain medium is more rapidly carried out. However, itshould be noted that a variation in decay rate does not always cause anincrease in extraction efficiency.

The decay rate is only an index indicating how fast an electron and ahole coupled in the light emitting layer can be converted into photoenergy. Accordingly, for the relation between the decay rate and theextraction efficiency, it is also necessary to take into considerationthe decay rate caused by nonradiative recombination of a gain medium inthe light emitting layer.

Although it is difficult to directly substitute a variation inextraction efficiency for a variation in decay rate, it is possible toanalogize a qualitative relation that an increase in decay rate causes amore active radiative recombination, and thus, a reduction in theprobability of nonradiative recombination, so that an enhancement inextraction efficiency will be induced.

Hereinafter, the effect of the photonic crystal when the gap between themirror and the light emitting layer falls under the reinforcedinterference condition will be described. The reinforced interferencecondition is established when the gap corresponds to about ¾(λ/n). Theradiation pattern generated under the reinforced interference conditionis shown in FIG. 24. When this radiation pattern is compared with thatof FIG. 10, it can be seen that a relatively large amount of lightpropagates in a vertical direction.

The results of a measurement for an extraction efficiency enhancementobtained when a photonic crystal is introduced under the above-describedcondition are shown in FIG. 25.

For a structure to which no photonic crystal is applied, the reinforcedinterference condition provides an extraction efficiency enhancementcorresponding to about 2 times that of the spherical wave conditionproviding no or little mirror effect. The reason why the extractionefficiency enhancement in this case is higher than the extractionefficiency enhancement calculated in association with the calculation ofthe interference effect by the mirror (increased by 1.6 times) is thatthe absorbance was taken into consideration in this structure.

When graphs associated with the cases, to which a photonic crystal(periodicity=800 nm, and etching depth=225 nm) is applied, are compared,it can be seen that the structure, to which the reinforced interferencecondition is applied, exhibits best characteristics.

Of course, in the structure, to which the reinforced interferencecondition is applied, the relative enhancement exhibited across thephotonic crystal is a maximum of about 1.2 times. This is because alarge portion of the light generated in the light emitting layer isinitially within the critical angle, so that the amount of lightextracted through the photonic crystal is correspondingly reduced.

The results of a measurement for a variation in extraction efficiencydepending on the periodicity of the photonic crystal under thereinforced interference condition are shown in FIG. 26.

In this case, the etching depth of the photonic crystal was fixed to 225nm. Also, the size of holes forming the photonic crystal was 0.25a. Inorder to identify the dependency of the extraction efficiency on theperiodicity of the photonic crystal under the reinforced interferencecondition and under a normal condition, two results thereof weredepicted in one graph.

Referring to the results, it can be seen that there is no remarkabledifference between the optimal periodicity of the photonic crystal underthe reinforced interference condition and the optimal periodicity of thephotonic crystal under the normal condition, and the optimalperiodicities are in the vicinity of 800 nm.

Hereinafter, a variation in extraction efficiency depending on theetching depth of the photonic crystal will be described with referenceto FIG. 27.

Under a normal condition in which the output pattern of the lightemitting layer may be assumed as a spherical wave, the periodicity ofthe photonic crystal can be divided, with reference to a periodicity ofabout him, into a periodicity, in which the extraction efficiency issaturated at a certain etching depth, and a periodicity, in which theextraction efficiency increases continuously in proportion to anincrease in etching depth.

This is because the photonic crystal can more effectively diffractfully-reflected light near the critical angle when the photonic crystalhas a longer periodicity. When this principle is applied to thecurrently-discussed reinforced interference condition, it can beexpected that the function of the photonic crystal having a longerperiodicity becomes more important because radiation of light in avertical direction is initially carried out under the above-describedcondition.

In order to verify this effect, the extraction efficiency according tothe periodicity was calculated through a calculation using a computersimulation, while varying the etching depth, as shown in FIG. 27. Aftera comparison of this condition with the normal condition, it can be moreclearly seen that the optimal periodicity, in which a maximum extractionefficiency is obtained, is shifted in a longer-periodicity direction asthe etching depth increases.

For example, when the etching depth is 900 nm, the optimal periodicityis found at 2 μm or more. This corresponds to a structure which can bemanufactured using the resolution of the current generalphotolithography, so that it is highly significant in terms of practicaluse.

First Embodiment

As shown in FIG. 28, a light extracting layer 120 is formed over aGaN-based semiconductor layer 110 previously formed over a substrate100, using a material having a refractive index similar to or higherthan that of the semiconductor layer 110.

The light extracting layer 120 may have a specific pattern. The specificpattern may form a photonic crystal having a periodic structure. Theformation of such a photonic crystal may be achieved through an etchingmethod or other patterning methods.

For the formation of the photonic crystal structure, a positivelithography for forming holes 121 or a negative lithography for formingrods is usable.

The photonic crystal pattern may be formed by forming the lightextracting layer 120 in accordance with a deposition process, and thensubjecting the light extracting layer 120 to a lithography process andan etching process. Alternatively, the photonic crystal pattern may beformed by performing a lithography on the semiconductor layer 110,depositing the light extracting layer 120, and then performing alift-off process.

When the above-described photonic crystal structure is arranged on asurface of the light emitting device, it is possible to extract lightconfined due to full reflection, through a diffraction procedure, andthus, to achieve an enhancement in extraction efficiency. However, evenin the case in which the light extracting layer 120 has a specificpattern as described above, it is also possible to enhance theextraction efficiency by forming a rough surface.

A transparent electrode material may be formed in the holes 121 of thelight extracting layer 120, which form the photonic crystal. For thetransparent electrode material, a transparent conducting oxide (TCO) 130may be used.

For the transparent conducting oxide 130, indium tin oxide (ITO) may beused. Also, indium zinc oxide (IZO), aluminum zinc oxide (AZO),magnesium zinc oxide (MZO), or gallium zinc oxide (GZO) may be used.

When the semiconductor layer 110 is a gallium nitride layer, the lightextracting layer 120 may have a refractive index of about 2.4 or morebecause the refractive index of gallium nitride is 2.4. The refractiveindex of the light extracting layer 120 may also be slightly lower than2.4.

For the light extracting layer 120, an oxide or a nitride may also beused. In particular, SiN or TiO₂ may be used.

FIG. 29 illustrates the structure of a horizontal light emitting devicehaving the above-described light extracting layer 120. The semiconductorlayer 110 may include an n-type semiconductor layer 111, an active layer112, and a p-type semiconductor layer 113 which are sequentially formedover the substrate 100 in this order. Also, the substrate 100 is made ofsapphire having a refractive index of 1.78. If necessary, a buffer layer114 may be interposed between the substrate 100 and the n-typesemiconductor layer 111.

The p-type semiconductor layer 113 arranged adjacent to the lightextracting layer 120 may completely maintain the thickness obtained uponlaminating the layer 113, without being etched upon patterning the lightextracting layer 120. The thickness of the p-type semiconductor layer113 may be 30 to 500 nm. Also, the light extracting layer 120 may have athickness of 150 nm or more.

In FIG. 29, the pattern of the light extracting layer 120 is shown in anenlarged state. The pattern of the light extracting layer 120 isconstituted by a plurality of holes 121 forming a photonic crystal in aGaN semiconductor. The radius, depth, and periodicity of the holes 121may be optimized for the associated semiconductor layer 110.

That is, when it is assumed that the periodicity of the holes 121,namely, the spacing between adjacent holes 121, is “a”, the radius ofeach hole 121 may be 0.1a to 0.45a, and the depth of each hole 121 maybe 0.25λ/n to 10λ/n. Here, represents the wavelength of emitted light,and “n” represents the refractive index of the medium, at which thephotonic crystal is formed, namely, the refractive index of the p-typesemiconductor layer 113. Also, the periodicity “a” may be 200 nm to5,000 nm.

Meanwhile, it may be possible to form the photonic crystal structure byforming regular rods, in place of the holes 121, as described above.

In order to form an n-type electrode 140 on the n-type semiconductorlayer 111, an etching process may be performed for the semiconductorlayer 110 such that the n-type semiconductor layer 111 is exposed at oneside thereof. A p-type electrode 150 may be formed on a region where thelight extracting layer 120 is formed.

The thickness of the material forming the light extracting layer 120 canbe freely determined. The structure, in which a material having a highrefractive index is deposited to form a photonic crystal, is applicableto any types of light emitting devices emitting red, green, or othercolors.

In order to identify the effects of the present invention, a variationin extraction efficiency depending on the refractive index of the lightextracting layer 120 was measured for the structure according to thefirst embodiment, as shown in FIG. 30.

In the graph of FIG. 30, the vertical axis represents a relativeenhancement in extraction efficiency in a general planar structure, inwhich no light extracting structure is introduced.

Referring to the results depicted in the graph, it can be seen that theextraction efficiency enhancement increases as the difference betweenthe refractive index of the transparent conducting oxide 130 and therefractive index of the light extracting layer 120 increases. In FIG.30, the broken line represents the extraction efficiency in the case inwhich the uppermost layer of the semiconductor layer (the p-typesemiconductor layer in this case) is etched to form a photonic crystal.

When the refractive index of the light extracting layer 120 is in thevicinity of 2.6, an extraction efficiency enhancement similar to that ofthe case, in which the p-type GaN semiconductor layer is etched, isexhibited. When the light extracting layer 120 or photonic crystal isformed using a material having a higher refractive index, a superiorextraction efficiency enhancement effect appears, as compared to theabove-described photonic crystal structure formed in accordance withetching of the p-type semiconductor layer.

Therefore, the condition to be given to the light extracting layer 120is that the light extracting layer 120 should have a refractive indexsimilar to or higher than the refractive index of the semiconductorlayer 20 (2.4), and should have a thickness of at least 150 nm (λ/n). Ifnecessary, the thickness of the light extracting layer 120 may have athickness of at least λ/4n.

Also, the material of the light extracting layer 120 should not causeabsorption loss within the wavelength range of the light emitting layerin the light emitting device. The material of the light extracting layer120 should also have an excellent physical bonding force.

For a material satisfying the above conditions, a silicon nitride(Si₃N₄) having a refractive index of about 2.4 or a titanium oxide(TiO₂) having a refractive index of 3.0 is preferred.

When the light extracting layer 120 is formed over the semiconductorlayer 110, using a material having a refractive index similar to orhigher than the refractive index of the semiconductor layer 110, asdescribed above, it is possible to maintain desired electricalcharacteristics of the light emitting device while introducing aphotonic crystal into the light emitting device, for an enhancement inextraction efficiency. Also, the same light extraction effect can beexhibited even for a higher current.

Moreover, it is possible to realize a light extraction effect equal toor superior to that of the case in which a GaN semiconductor layer isetched to form a photonic crystal and to increase the etching depth uponforming the photonic crystal to a level causing a saturation of theextraction efficiency.

Second Embodiment

As shown in FIG. 31, a GaN-based semiconductor layer 201 is formed overa sapphire substrate 200 having a refractive index of 1.78. Atransparent conducting layer 220 may be formed over the semiconductorlayer 210. The transparent conducting layer 220 can be used as anelectrode.

For the transparent conducting layer 220, indium tin oxide (ITO) may beused. Also, indium zinc oxide (IZO), aluminum zinc oxide (AZO),magnesium zinc oxide (MZO), or gallium zinc oxide (GZO) may be used.

A light extracting layer 230 is formed on the transparent conductinglayer 220, using a material having a refractive index similar to orhigher than the refractive index of the semiconductor layer 210.

The light extracting layer 230 may have a specific pattern. The specificpattern may form a photonic crystal having a periodic hole structure.The formation of such a photonic crystal may be achieved through anetching method or other patterning methods.

When the semiconductor layer 210 is a gallium nitride layer, the lightextracting layer 230 may have a refractive index of about 2.4 or morebecause the refractive index of gallium nitride is 2.4. The refractiveindex of the light extracting layer 230 may also be slightly lower than2.4. For the light extracting layer 230, an oxide or a nitride may alsobe used. In particular, SiN or TiO₂ may be used. Such matters may beidentical to those of the first embodiment.

Meanwhile, as shown in FIG. 32, a thin transparent metal layer 240consisting of, for example, an Ni layer 241 and an Au layer 242, may beused, in place of the transparent conducting layer 220. The transparentmetal layer 240 is sufficiently thin to allow light to pass therethough.

Of course, the transparent metal layer 240 may be formed using an alloycontaining Ni or Au.

Preferably, the transparent metal layer 240 has a thickness of severalnanometers less than 10 nm. For example, the Ni layer 241 may have athickness of 2 nm or less, and the Au layer 242 may have a thickness of4 nm or less.

As described above, the transparent conducting layer 220 or transparentmetal layer 240 may be formed over the semiconductor layer 210. Wherethe transparent conducting layer 220 is formed using a metal oxide, itis preferred that the thickness of the transparent conducting layer 220be sufficiently small.

FIG. 33 illustrates the transmittance depending on the incidence angleof light for the transparent conducting layer 220, which is made of, forexample, ITO, and the transparent metal layer 240. Referring to FIG. 33,it can be seen that, when ITO is used, a great reduction intransmittance occurs at an incidence angle larger than about 45°.

Also, referring to FIG. 34, it can be seen that the extractionefficiency enhancement obtained by the light extracting layer 230 isgradually reduced as the thickness of the ITO layer increases.Accordingly, when the light extracting layer 230 has a refractive indexlower than that of the semiconductor layer 210, as described above, itis preferred that the thickness of the transparent conducting layer 220be smaller than λ/2n (“λ” represents the wavelength of light, and “n”represents the refractive index of the transparent conducting layer).

Since the transparent conducting layer 220 can function as an electrode,it is more advantageous that the transparent conducting layer 220 has athickness of λ/16n to λ/4n, when voltage characteristics are taken intoconsideration.

In the case of the transparent metal layer 240, it is possible todetermine the thickness of the transparent metal layer 240 because adesired transmittance can be maintained at most angles, as shown in FIG.33.

FIG. 35 illustrates a light emitting device having the above-describedstructure. In the illustrated case, the semiconductor layer 210 includesan n-type semiconductor layer 211 arranged on the sapphire substrate200, a light emitting layer 212 arranged on the n-type semiconductorlayer 211, and a p-type semiconductor layer 213 arranged on the lightemitting layer 212. An undoped low-temperature buffer layer 214 may beinterposed between the sapphire substrate 200 and the n-typesemiconductor layer 211.

A current diffusion layer 215 having a thickness of several nanometersmay be formed over the p-type semiconductor layer 213. The currentdiffusion layer 215 may be made of an undoped semiconductor layer.

In particular, for the current diffusion layer 215, an In_(x)Ga_(1-x)Nlayer or an In_(x)Ga_(1-x)N/GaN superlattice layer may be used. Thecurrent diffusion layer 215 can function to enhance mobility ofcarriers, and thus to cause current to flow smoothly. In this regard,such a current diffusion layer is also called a current transportenhanced layer (CTEL).

The p-type semiconductor layer 213 may have a thickness of 30 to 500 nm.Also, the light extracting layer 230 may have a thickness of λ/4n ormore. Here, “n” represents the refractive index of the material formingthe light extracting layer 230.

Matters not described in association with this embodiment may beidentical to those of the first embodiment.

Third Embodiment

FIG. 36 illustrates an embodiment in which a light extracting layer 320having a high refractive index is applied to the structure of a verticallight emitting device.

In this embodiment, the light extracting layer 320 is formed over asemiconductor layer 310, using a material having a refractive indexsimilar to or higher than the refractive index of the semiconductorlayer 310. The semiconductor layer 310 includes an n-type semiconductorlayer 311, a light emitting layer 312 arranged on the n-typesemiconductor layer 311, and a p-type semiconductor layer 313 arrangedon the light emitting layer 312.

The light extracting layer 320 may have a specific pattern. The specificpattern may form a photonic crystal having a periodic structure. Theformation of such a photonic crystal may be achieved through an etchingmethod or other patterning methods.

For the formation of the photonic crystal structure, a positivelithography for forming holes or a negative lithography for forming rodsis usable. This matter may be identical to that of the first embodiment.

As shown in FIG. 36, the semiconductor layer 310 may be formed over anelectrode consisting of a transparent electrode 330 having ohmiccharacteristics and a reflective electrode 340. The transparentelectrode 330 may be formed using a conducting oxide such as ITO, andthe reflective electrode 340 may be formed using a metal such as Al orAg.

The electrode may also consist of a single-layer reflective ohmicelectrode (not show), in place of the multi-layer structure consistingof the transparent electrode 330 and refractive electrode 340.

The above-described structure may be arranged on a support layer 350made of a metal or semiconductor layer. In this case, the support layer350 can support the light emitting device structure in a procedure forremoving a substrate, on which the semiconductor layer 310 has beengrown, to obtain a vertical structure.

In this vertical light emitting device structure, an n-type electrode isformed on the light extracting layer 320 such that the n-type electrodeis electrically connected to the n-type semiconductor layer 313. Thus,current flows vertically during operation of the light emitting device.

Fourth Embodiment

FIG. 37 illustrates the structure of a photonic crystal layer 420 inwhich photonic crystals having different periodicities are mixed. FIGS.38 and 39 are electro-microscopic photographs of structures obtainedwhen the photonic crystal layer 420 is experimentally implemented.

Referring to the electro-microscopic photographs, it can be seen that,when the surface of the n-type GaN semiconductor layer in the verticalstructure is patterned in accordance with a general etching process, afine pattern is additionally formed as the surface of the GaNsemiconductor layer reacts with plasma of the gas used in the etchingprocess.

The photonic crystal layer 420 formed in accordance with theabove-described formation procedure has a periodicity-mixed structureincluding a periodic photonic crystal structure and an additional randomstructure having an average periodicity shorter than that of theperiodic photonic crystal structure.

In order to arithmetically evaluate the effect of the periodicity-mixedstructure of the photonic crystal layer 420, an extraction efficiencycomparison was conducted for structures of FIGS. 40 and 41, through acalculation using a computer simulation.

The periodicity-mixed structure of the photonic crystal layer 420 may bevariously expressed. However, for the simplification of the expression,the periodicity-mixed structure was expressed in accordance with thefollowing principle.

First, the structure of a first photonic crystal 421 having a relativelylong periodicity as a first periodicity was introduced. The etchingdepth of the structure of the first photonic crystal 421 was set to 450nm.

The structure of a second photonic crystal 422 having a relatively shortperiodicity as a second periodicity was introduced into a portion of thelong-periodicity structure of the first photonic crystal 421 which wasnot etched. The etching depth of the short-periodicity structure of thesecond photonic crystal 422 was set to 225 nm.

The structure expression was experimentally conducted in a calculationspace in accordance with a method in which the second photonic crystal422 having a short periodicity is first defined, and then the firstphotonic crystal 421 having a long periodicity is introduced.

In this case, the periodicity, etching depth, and shape of eachstructure of the mixed photonic crystals 421 and 422 may be variouslyexpressed. Accordingly, various periodicity-mixed structures may beconceived for the photonic crystal layer 420 in accordance with variouscombinations of the above-described structural factors.

Referring to the results of the calculation, it can be seen that thephotonic crystal structure, in which different periodicities are mixed,always exhibits a superior extraction efficiency enhancement effect, ascompared to other structures, as shown in FIG. 42. Therefore, if amethod capable of reliably manufacturing an experimentalperiodicity-mixed photonic crystal structure can be provided, it ispossible to expect an extraction efficiency improved over that of asingle photonic crystal structure, irrespective of a combination ofphotonic crystal structures.

As described above, when a photonic crystal is formed by etching ann-type GaN semiconductor layer having a thickness of 3 μm uponintroducing the photonic crystal into a vertical GaN-based lightemitting device, it is possible to maintain desired electricalcharacteristics, as compared to a horizontal structure, in which aphotonic crystal is formed at a p-type GaN semiconductor layer. Also,there is no substantial limitation on etching depth.

The extraction efficiency enhancement effect depending on the etchingdepth in a photonic structure having a single periodicity can besummarized as follows. That is, when the etching depth of the photoniccrystal introduced into the n-type GaN semiconductor layer is 300 nm ormore, and the periodicity of the introduced photonic crystal is fpm ormore, but less than 5 μm, the photonic structure, which satisfies theabove two conditions, exhibit a tendency that that the extractionefficiency approaches a maximum extraction efficiency while increasingcontinuously in proportion to the etching depth.

As the etching depth increase, the optimal periodicity is shifted to alonger-periodicity direction. For example, the optimal photonic crystalperiodicity is in the vicinity of 800 nm at an etching depth of 225 nm,but is 1,400 nm at an etching depth of 900 nm.

When there is no substantial limitation on etching depth, variousperiodicity-mixed photonic crystal structures can be proposed. The shapeof the periodicity-mixed photonic crystal structure may be classified,in accordance with the manufacturing method thereof, as follows.

As shown in FIG. 43, the periodicity-mixed photonic crystal layer 420may be formed over the semiconductor layer 410 by forming the firstphotonic crystal 421, which has a relatively long periodicity, inaccordance with an etching process, and then forming the second photoniccrystal 422, which has a relatively short periodicity, in accordancewith an etching process. In this case, the second photonic crystal 422has a random structure having an average periodicity shorter than theperiodicity of the first photonic crystal 421.

When the short-periodicity second photonic crystal 422 is formed afterthe formation of the long-periodicity first photonic crystal 421, asdescribed above, the second photonic crystal 422 is also formed in holes421 a forming the first photonic crystal 421. Thus, the second crystal422 can be present in the overall portion of the light emitting surface.

The periodicity of the first photonic crystal 421 corresponding to thelongest periodicity of the photonic crystal layer 420 may be 800 to5,000 nm. The depth of a pattern forming the first photonic crystal 421,which has a periodicity corresponding to the longest periodicity of thephotonic crystal layer 420, may be 300 to 3,000 nm.

The periodicity of the second photonic crystal 422 corresponding to theshortest periodicity of the photonic crystal layer 420 may be 50 to1,000 nm. The depth of a pattern forming the second photonic crystal422, which has a periodicity corresponding to the shortest periodicityof the photonic crystal layer 420, may be 50 to 500 nm.

Meanwhile, when it is assumed that the periodicity of the photoniccrystal layer 420 is “a”, the depth of holes forming the photoniccrystals of the photonic crystal layer 420 may be 0.1a to 0.45a.

FIG. 44 illustrates a structure of the periodicity-mixed photoniccrystal layer 420 formed over the semiconductor layer 410, in which thesecond photonic crystal 422, which has a relatively short periodicity,in accordance with an etching process, and then the first photoniccrystal 421, which has a relatively long periodicity, is formed inaccordance with an etching process.

Alternatively, as shown in FIG. 45, the first photonic crystal 421,which has a relatively long periodicity, may be first formed inaccordance with an etching process, and then the second photonic crystal422, which has a relatively short periodicity, may be formed inaccordance with a deposition process.

When the second photonic crystal 422 is formed in accordance with adeposition process, it has a pattern of particles 422 a protruded fromthe structure of the first photonic crystal 421, in place of anengraving pattern. The particles 422 a may have a hemispherical shape.The particles 422 a may also have a hexagonal structure in accordancewith the shape of GaN crystals.

When the short-periodicity second photonic crystal 422 is formed afterthe formation of the long-periodicity first photonic crystal 421, asdescribed above, the second photonic crystal 422 is also formed in holes421 a forming the first photonic crystal 421. Thus, the second crystal422 can be present in the overall portion of the light emitting surface.

FIG. 46 illustrates a structure in which the second photonic crystal422, which has a relatively short periodicity, in accordance with adeposition process, and then the first photonic crystal 421, which has arelatively long periodicity, is formed in accordance with an etchingprocess.

Since the above-described periodicity-mixed photonic crystal layer 420has a structure including photonic crystals 421 and 422 having differentperiodicities, various periodicity-mixed structures may be conceived forthe photonic crystal layer 420 in accordance with various combinationsof the structural factors of the photonic crystals 421 and 422.

Basically, the extraction efficiency depends on a combination of theperiodicities of the two photonic crystals 421 and 422. In this case,the etching depth, photonic crystal shape, etc. may function asparameters. When a new material is introduced due to the depositionprocess, the refractive index of the introduced material may alsofunction as a parameter.

When a photonic crystal is introduced into an n-type GaN semiconductorlayer in a vertical GaN-based light emitting device, for an enhancementin external extraction efficiency, it is possible to secure a maximumextraction efficiency in a long-periodicity structure (1 μm), which iseasily manufactured, by deeply etching the photonic crystal inaccordance with the present invention.

In accordance with the present invention, it is possible to maximize theextraction efficiency enhancement by providing a periodicity-mixedphotonic crystal structure, in which two or more periodicities are mixedon the same plane, as an extension from a photonic crystal structurehaving a single periodicity.

FIG. 47 illustrates the structure of a vertical light emitting devicewhich includes the above-described periodicity-mixed photonic crystallayer 420.

The semiconductor layer 410, in which the above-described photoniccrystal structure is formed, includes an n-type semiconductor layer 411,a light emitting layer 412, and a p-type semiconductor layer 413 whichare arranged in this order. As described above, the photonic crystallayer 420 is formed on the n-type semiconductor layer 411.

An ohmic electrode layer or reflective ohmic electrode layer 430 may bearranged beneath the semiconductor layer 410. The above-described lightemitting device structure may be arranged on a support layer 440 made ofa semiconductor such as silicon or a metal.

An n-type electrode 450 may be arranged on the n-type semiconductorlayer 411 formed with the photonic crystal layer 420.

The above-described structure has a feature in that the photonic crystalhas no or little effect to cause an increase in resistance in thesemiconductor layer 410 because it is formed in accordance with etchingof the n-type GaN semiconductor layer 411. Also, the light extractioneffect obtained in accordance with the introduction of the photoniccrystal layer 420 can be equally maintained even in a high-power outputregion because the vertical GaN-based light emitting device can easilydischarge heat.

Meanwhile, since the thickness of the n-type GaN semiconductor layer 411can generally be larger than 3 μm, the etching depth of the photoniccrystal can be increased to be considerably larger than the etchingdepth at which the extraction efficiency is saturated.

As the etching depth of the photonic crystal increases as describedabove, the periodicity capable of securing a maximum extractionefficiency is shifted in a longer-periodicity direction. In particular,the photonic crystal, which has a periodicity of 1 μm or more, cancontinuously increase the extraction efficiency for an etching depthlarger than an etching depth at which the extraction efficiency issaturated when a photonic crystal having a shorter periodicity is used.

Meanwhile, the structure of the periodicity-mixed photonic crystal layer420, in which different periodicities are mixed in the same plane, asdescribed above, can exhibit a superior extraction efficiencyenhancement, irrespective of the combination of photonic crystals, ascompared to the structure having a single periodicity. Also, the shiftof the optimal periodicity depending on the etching depth and thestructure of the periodicity-mixed photonic crystal layer 420 areapplicable to other light emitting device structures in which thethickness of a photonic crystal forming layer is 300 nm or more.

Fifth Embodiment

FIG. 48 illustrates the structure of a light emitting device which iscapable of maximizing the extraction efficiency, taking intoconsideration the effect of a photonic crystal exhibited when the gapbetween a mirror and a light emitting layer falls under a reinforcedinterference condition.

In the illustrated light emitting device structure, a reflectiveelectrode 530 is arranged on a support layer 540, and a semiconductorlayer 510 is arranged on the reflective electrode 530. For thereflective electrode 530, a material capable of making an ohmic contactwith the semiconductor layer 510 may be used. Preferably, the reflectiveelectrode 530 has a reflectance of 50% or more.

As shown in FIG. 49, a separate ohmic electrode 531 may be interposedbetween the reflective electrode 530 and the semiconductor layer 510. Inthis case, for the ohmic electrode 531, a transparent electrode may beused. For the transparent electrode, indium tin oxide (ITO) having a lowrefractive index may be used. Also, indium zinc oxide (IZO), aluminumzinc oxide (AZO), magnesium zinc oxide (MZO), or gallium zinc oxide(GZO) may be used.

As shown in FIG. 48 or 49, the semiconductor layer 510 includes a p-typesemiconductor layer 513, a light emitting layer 512 arranged on thep-type semiconductor layer 513, and an n-type semiconductor layer 511arranged on the light emitting layer 512.

A photonic crystal 520, which consists of a pattern including aplurality of holes 521 or rods, may be arranged on the n-typesemiconductor layer 511. An n-type electrode 550 is arranged on aportion of the n-type semiconductor layer 511. The pattern of thephotonic crystal 520 may not be formed on a region where the n-typeelectrode 550 is arranged.

In the photonic crystal 520, the depth of each hole 521 or the height ofeach rod may be 300 to 3,000 nm. The photonic crystal 520 may have aperiodicity of 0.8 to 5 μm. When it is assumed that the periodicity ofthe photonic crystal 520 is “a”, the size (diameter) of each hole 521 oreach rod may be 0.25 to 0.45a.

As described above, the distance “d” between the reflective electrode530 and the center of the light emitting layer 512 may be 0.65λ/n to0.85λ/n, or may be an odd multiple of λ/4n.

The distance between the reflective electrode 530 and the light emittinglayer 512 may be adjusted consequently by the p-type semiconductor layer513. That is, it is possible to establish a reinforced interferencecondition in a light extraction procedure by adjusting the distancebetween the reflective electrode 530 and the light emitting layer 512.

When a separate transparent ohmic electrode 531 made of a material suchas ITO is interposed between the reflective electrode 530 and the lightemitting layer 512, it is possible to more easily carry out theprocedure for adjusting the thickness of the p-type semiconductor layer513 to fall under the reinforced interference condition.

FIG. 50 depicts the extraction efficiency depending on the thickness ofthe ohmic electrode 531 when ITO is used for the ohmic electrode 531 inthe structure of FIG. 49, using a graph. FIG. 51 depicts a variation inextraction efficiency depending on a variation in the thickness of thep-type semiconductor layer 513 in the structure of FIG. 49.

Referring to FIGS. 50 and 51, it can be seen that the extractionefficiency control can be more easily achieved by adjusting thethickness of the transparent ohmic electrode 531. This means that it ispossible to more easily adjust the reinforced interference condition forlight extraction by controlling the thickness of the ohmic electrode531.

In accordance with the present invention, when the photonic crystal 520is introduced into the n-type semiconductor layer 511 of the verticalGaN-based light emitting device, in order to achieve an enhancement inexternal extraction efficiency, it is possible to obtain a maximumextraction efficiency under the condition in which the photonic crystal520 has a long periodicity (1 μm or more) such that it is easilymanufactured, using the interference effect of the reflective electrode530 and the etching depth. Also, it is possible to obtain an enhancementin extraction efficiency, using only the interference effect of thereflective electrode 530.

When the above-described light emitting device structure is packaged, itcan exhibit a high extraction efficiency, irrespective of the structureof the package.

FIG. 52 illustrates a package structure in which leads 561 are formedaround a package body 560, and a light emitting device 500 having theabove-described features is mounted on the top of the package body 560.The light emitting device 500 may be connected to the leads 561 viawires 562. A zener diode 570 is arranged at one side of the lightemitting device 500, to achieve an improvement in withstanding voltagecharacteristics.

The package body 560, on which the light emitting device 500 is mounted,is encapsulated with a planar encapsulate.

FIG. 53 illustrates a structure in which the light emitting device 500is mounted on a package body 580, and a dome-shaped encapsulate 590 isformed.

Light emitted from the above-described light emitting device 500 whilehaving a strong directionality in a vertical direction can exhibitsubstantially-equal power characteristics in the planar packagestructure of FIG. 52 and the dome-shaped package structure of FIG. 53.

Sixth Embodiment

Hereinafter, a procedure for manufacturing a light emitting devicehaving a light extracting structure, such as a photonic crystal, inaccordance with the present invention will be described.

FIG. 54 illustrates an LED structure 600 formed on a substrate 610.

For the formation of the LED structure 600, a compound semiconductorlayer 620 is first formed over the substrate 610 which is made of, forexample, sapphire. The semiconductor layer 620 includes an n-typesemiconductor layer 621, an active layer 622, and a p-type semiconductorlayer 623 which are arranged, in this order, starting from the side ofthe substrate 610.

The arrangement order of the n-type semiconductor layer 621, activelayer 622, and p-type semiconductor layer 623 may be reversed. That is,these layers may be formed over the substrate 610 in the order of thep-type semiconductor layer 623, the active layer 622, and the n-typesemiconductor layer 621.

In particular, for the semiconductor layer 620, a GaN-basedsemiconductor may be used. In this case, the active layer 622 may havean InGaN/GaN quantum well (QW) structure. Also, a material such as AlGaNor AlInGaN may be used for the active layer 622. when an electric fieldis applied to the active layer 622, light is generated in accordancewith coupling of electron-hole pairs.

In order to achieve an enhancement in brightness, the active layer 622may have a multi-quantum well (MQW) structure including a plurality ofQW structures as described above.

A p-type electrode 630 is formed over the semiconductor layer 620. Thep-type electrode 630 is an ohmic electrode. A reflective electrode 640may be formed over the p-type electrode 630, to reflect light generatedfrom the active layer 622, and thus, to externally emit the generatedlight.

In place of the p-type electrode 630 and reflective electrode 640, asingle electrode may be formed by appropriately selecting the materialsof the p-type electrode 630 and reflective material 640, to function asboth the p-type electrode 630 and the reflective electrode 640.

A support layer 650 may be formed over the reflective electrode 640, tosupport the LED structure 600 in a subsequent procedure for separatingthe substrate 610.

The support layer 650 may be formed by bonding a semiconductor substratemade of silicon (Si), gallium arsenide (GaAs), or germanium (Ge) or ametal substrate made of CuW to an upper surface of the reflectiveelectrode 640. Alternatively, the support layer 650 may be formed byplating a metal such as nickel (Ni) or copper (Cu) over the reflectiveelectrode 640.

Where the support layer 650 is made of a metal, it may be formed using aseed metal, in order to enhance the bondability to the reflectiveelectrode 640.

In accordance with the above-described processes, the LED structure 600has a structure as shown in FIG. 54. From this structure, the substrate610 is then removed. Thereafter, a dielectric layer 660 is formed overthe surface, from which the substrate 610 has been removed. Thus, astructure as shown in FIG. 55 is obtained.

The removal of the substrate 610 may be achieved in accordance with alaser lift-off process using a laser. Alternatively, the substrate 610may be removed in accordance with a chemical method such as an etchingmethod.

During the procedure for removing the substrate 610, the support layer650 supports the LED structure 600.

As described above, the dielectric layer 660 is formed over the n-typesemiconductor layer 621 exposed in accordance with the removal of thesubstrate 610. For the dielectric layer 660, an oxide or a nitride maybe used. For example, a silicon oxide (SiO₂) may be used.

A plurality of regular holes 661 are formed in the dielectric layer 660.The formation of the holes 661 may be achieved using a dry etchingprocess such as a reactive ion etching (RIE) process or aninductively-coupled plasma reactive ion etching (ICP-RIE) process.

The dry etching process is suitable for the formation of the holes 661because uni-directional etching can be achieved, different from a wetetching process. That is, although isotropic etching is carried out inthe wet etching process such that etching is achieved in all directions,etching only in a depth direction for formation of the holes 661 can beachieved in accordance with the dry etching process. Accordingly, theholes 661 can be formed to have a desired pattern in terms of size andspacing, in accordance with the dry etching process.

In order to form the multiple holes 661 as described above, a patternmask 670 formed with a hole pattern 671 as shown in FIG. 56 may be used.

For the pattern mask 670, a metal mask made of, for example, chromium(Cr), may be used. If necessary, photoresist may be used.

Where photoresist is used for the pattern mask 670, the hole pattern 671may be formed using a photolithography, an e-beam lithography,nano-imprinted lithography, or the like. In this procedure, a dryetching process or a wet etching process may be used.

Where a chromium mask is used for the pattern mask 670, a polymer layeris first formed a chromium layer, to form a pattern on the chromiumlayer. A pattern is then formed over the polymer layer in accordancewith an imprinting method. Thereafter, the chromium layer is etched.Thus, the pattern mask 670 is formed. The etching of the chromium layeris achieved using a dry etching process.

For the dry etching process, an RIE process or an ICP-RIE process may beused. For the gas used in this process, at least one of Cl₂ and O₂ maybe used.

Preferably, the hole pattern 671 is not formed in a certain region, toprovide an empty space where n-type electrode pads 691 (FIG. 66) will beformed.

Through the above-described procedure, a plurality of holes 661 havingthe same pattern as the hole pattern 671 are formed in the dielectriclayer 660, as shown in FIG. 57. In this case, the holes 661 extendthroughout the dielectric layer 660.

The holes 661 may have various patterns. For example, the holes 661 havea square pattern. Also, the holes 661 may have various patterns as shownin FIGS. 58 to 62.

That is, the holes 661 are formed to be arranged along oblique linesdefined in a light emitting device package, as show in FIG. 58. Also,the holes 661 may be formed to be arranged along oblique lines definedin a plurality of divided surface regions in a light emitting devicepackage, as shown in FIG. 59. In this case, the oblique-line pattern ofeach surface region may not meet those of the remaining surface regions.

Alternatively, the oblique-line pattern of each region may meet those ofthe remaining regions, as shown in FIGS. 60 and 61. FIG. 60 illustratesthe case in which the holes 661 are arranged along oblique lines definedin two divided regions of a light emitting device such that theoblique-line patterns meet at the boundary of the regions. FIG. 61illustrates the case in which the holes 661 are arranged along obliquelines defined in four divided regions of a light emitting device suchthat the oblique-line patterns meet at the boundaries of the regions.

On the other hand, as shown in FIG. 62, the holes 661 may be arranged toform a plurality of concentric circle patterns or radial patterns.

In addition, the holes 661 may have various polygonal patterns such ashexagonal patterns or octagonal patterns, or trapezoidal patterns. Also,an irregular pattern may be formed.

After the formation of the hole 661 in the dielectric layer 660 arrangedon the n-type semiconductor layer 621, etching is carried out for then-type semiconductor layer 621 in accordance with a dry etching process,to form a plurality of grooves 624, as shown in FIG. 58.

Thus, the dielectric layer 660 formed over the n-type semiconductorlayer 621 functions as a mask or a passivation film for etching then-type semiconductor layer 621.

In this case, the grooves 624 are formed to have the same pattern as theholes 661.

That is, the grooves 624 may be formed to have a pattern such as asquare pattern, a plurality of oblique-line patterns, a plurality ofoblique-line patterns divided into at least two regions, a plurality ofoblique-line patterns divided into at least two regions while extendingin opposite directions, a plurality of concentric patterns, polygonalpatterns, trapezoidal patterns, or radial patterns

FIG. 63 illustrates a procedure for forming the grooves 624 in then-type semiconductor layer 621, using an ICP-RIE apparatus.

For the ICP-RIE apparatus, a planar type or a solenoid type may be used.In FIG. 63, a planar type ICP-RIE apparatus is illustrated.

In the ICP-RIE apparatus, a copper coil 710 is arranged on a chamber 700including a grounded metal shield 701 and an insulating window 702covering the metal shield 701. Electric power from a radio-frequency(RF) power supplier 720 is applied to the coil 710. In this case, anelectric field should be formed at an appropriate angle, in order toinsulate the insulating window 702 by the RF power.

The LED structure 600 formed with the dielectric layer 660 with thepattern of holes 661 is laid on a lower electrode 730 arranged in thechamber 700. The lower electrode 730 is connected to a bias voltagesupplier 740 for supplying a bias voltage to bias the LED structure 600such that desired etching is achieved.

The bias voltage supplier 740 preferably supplies RF power and a DC biasvoltage.

A gas mixture including at least one of Ar, BCl₂, and Cl₂ is introducedinto the chamber 700 through a reactive gas port 703. At this time,electrons are injected into the chamber 700 through a top port 704.

The injected electrons strike neutral particles of the introduced gasmixture due to an electromagnetic field generated by the coil 710,thereby generating ions and neutral atoms to produce plasma.

The ions in the plasma are accelerated to move toward the LED structure600, by the bias voltage from the bias voltage supplier 740 supplied tothe electrode 730. The accelerated ions pass through the pattern of theholes 661 of the dielectric layer 660, together with the acceleratedelectrons, thereby forming the pattern of the grooves 624 in the n-typesemiconductor layer 621, as shown in FIG. 64.

In this case, the pressure of the chamber 700 is maintained at 5 mTorr.An He flow may also be used. Preferably, the chamber 700 is cooled to10° C. during the etching procedure.

For the RF power supplier 720 and bias voltage supplier 740, electricpower of 33 W and 230 W may be used, respectively.

For the formation of the holes 660 in the dielectric layer 661, theabove-described ICP-RIE apparatus may also be used in the same manner asdescribed above. In this case, the gas mixture may include at least oneof CF₄, Ar, and CHF₃. The RF power supplier 720 and bias voltagesupplier 740 may use electric power of 50 W and 300 W, respectively.

The holes 660 may be irregularly formed in the dielectric layer 661 inthe above-described procedure, to irregularly form the grooves 624 inthe n-type semiconductor layer 621. The irregular grooves 624 provide arough surface, through which light is extracted, to achieve anenhancement in extraction efficiency.

However, it is preferred that the pattern of the grooves 624 be regularto have a certain periodicity (FIGS. 58 to 62), and thus, to form aphotonic crystal structure 680 on the surface of the n-typesemiconductor layer 621.

FIG. 65 illustrates a scanning electron microscope (SEM) image of thephotonic crystal structure 680 formed on the n-type semiconductor layer621 in accordance with the above-described procedures.

The photonic crystal structure 680 is formed such that the periodicityof a photonic crystal is 0.5 to 1.5 μm, and the diameter of the grooves624 forming the photonic crystal corresponds to about 0.3 to 0.6 timesthe photonic crystal periodicity, taking into consideration therefractive index of GaN (2.6), the refractive index of an epoxy lens,which is included in the LED structure and from which light isextracted, (1.5), and the relation with a driving voltage.

The grooves 624 may have a depth corresponding to ⅓ or more of thethickness of the n-type semiconductor layer 621.

When the above-described photonic crystal structure 680 is formed, ithas a periodic refractive index arrangement. When the periodicity of thephotonic crystal structure 680 corresponds to about half of thewavelength of emitted light, a photonic band-gap is formed in accordancewith multi-scattering of photons by a photonic crystal lattice having aperiodic variation in refractive index.

In the photonic crystal structure 680, light has a property of effectiveemission in a certain direction. That is, a light prohibition zone isformed. Accordingly, there may be a phenomenon that light is extractedthrough regions other than the holes 624 forming the phonic crystalstructure 680 without entering or passing through the holes 624.

As described above, the n-type electrode pads 691 and p-type electrodepads 692 are formed at upper and lower surfaces of the LED structure 600formed with the photonic crystal structure 680, as shown in FIG. 66.Thus, the LED structure 600 is completely manufactured.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A light emitting device comprising: a semiconductor structure havingmultiple layers, the semiconductor structure having a first surface anda second surface; a light extraction layer comprising at least two lightextraction structures on the first surface of the semiconductorstructure, the light extraction structures having differentdistributions of unit structures; and a reflective structure on thesecond surface of the semiconductor structure, the reflective structurecomprising an ohmic contact layer.
 2. The light emitting deviceaccording to claim 1, wherein the unit structures comprise a pluralityof holes or a plurality of protrusions.
 3. The light emitting deviceaccording to claim 1, wherein an average distance between adjacent unitstructures of the light extraction structure corresponding to thelongest average distance among the light extraction structures is in therange of 800 to 5,000 nm.
 4. The light emitting device according toclaim 3, wherein a height or a depth of the unit structure is in therange of 300 to 3,000 nm.
 5. The light emitting device according toclaim 1, wherein an average distance between adjacent unit structures ofthe light extraction structure corresponding to the shortest averagedistance among the light extraction structures is in the range of 50 to1,000 nm.
 6. The light emitting device according to claim 5, wherein aheight or a depth of the unit structure is in the range of 50 to 500 nm.7. The light emitting device according to claim 1, wherein a height or adepth of the unit structure is in the range of 0.1a to 0.45a, where “a”represents an average distance between adjacent unit structures of thelight extraction structure.
 8. The light emitting device according toclaim 1, wherein the semiconductor structure comprises: a p-typesemiconductor layer; an active layer arranged on the p-typesemiconductor layer; and an n-type semiconductor layer arranged on theactive layer.
 9. The light emitting device according to claim 8, whereinthe light extraction layer is formed on the n-type semiconductor layer.10. The light emitting device according to claim 1 further comprising: asupport layer on the reflective structure.
 11. The light emitting deviceaccording to claim 10, wherein the support layer comprises asemiconductor or a metal.
 12. The light emitting device according toclaim 1, wherein the light extraction layer comprises: a first lightextraction structure having a first average distance between adjacentunit structures; and a second light extraction structure having a secondaverage distance between adjacent unit structures, wherein the firstaverage distance is longer than the second average distance.
 13. Thelight emitting device according to claim 12, wherein the first lightextraction structure comprises a plurality of holes.
 14. The lightemitting device according to claim 13, wherein the second lightextraction structure is formed in the holes of the first lightextraction structure.
 15. The light emitting device according to claim13, wherein the second light extraction structure is formed on theoutside of the holes of the first light extraction structure.
 16. Thelight emitting device according to claim 12, wherein the second lightextraction structure comprises a plurality of protrusions.
 17. The lightemitting device according to claim 12, wherein the second lightextraction structure overlaps the first light extraction structure. 18.The light emitting device according to claim 1, wherein the reflectivestructure is a single layer.
 19. The light emitting device according toclaim 1, wherein the light extraction structures are formed on the firstsurface of the semiconductor structure.
 20. The light emitting deviceaccording to claim 19, wherein the light extraction structures areformed by etching the first surface of the semiconductor structure. 21.The light emitting device according to claim 1, wherein the lightextraction layer comprises the same material as that of thesemiconductor structure.
 22. A light emitting device comprising: asemiconductor structure having multiple layers; and a light extractionlayer on the semiconductor structure, the light extraction layercomprising a first light extraction structure having a periodicdistribution of unit structures and a second light extraction structurehaving a random distribution of unit structures.
 23. The light emittingdevice according to claim 22, wherein the second light extractionstructure has an average distance between adjacent unit structuresshorter than a periodicity of the first extraction structure.
 24. Thelight emitting device according to claim 22 wherein the first lightextraction structure and the second light extraction structure areformed on the semiconductor structure.
 25. The light emitting deviceaccording to claim 24, wherein the first light extraction structure andthe second light extraction structure are formed by etching thesemiconductor structure.
 26. The light emitting device according toclaim 22, wherein a periodicity of the first light extraction structureis in the range of 800 to 5,000 nm.
 27. The light emitting deviceaccording to claim 22, wherein a height or a depth of the unit structureof the first light extraction structure is in the range of 300 to 3,000nm.
 28. The light emitting device according to claim 22, wherein anaverage distance between adjacent unit structures of the second lightextraction structure is in the range of 50 to 1,000 nm.
 29. The lightemitting device according to claim 22, wherein a height or a depth ofthe unit structure of the second light extraction structure is in therange of 50 to 500 nm.
 30. The light emitting device according to claim22, wherein the second light extraction structure overlaps the firstlight extraction structure.
 31. The light emitting device according toclaim 22, wherein the light extraction layer comprises the same materialas that of the semiconductor structure.